Known combustors for hydrogen-rich fuels can rely upon very high levels of dilution (with inert species, for example, N2 and/or steam) of diffusion flames. See, for example, WO-A1-2008/135362 or WO-A1-2008/155242. Derating (i.e., reducing flame temperatures) can also be used. See, for example, EP-A1-0 731 255 or EP-A1-0 899 438. Efforts are being made to develop lean-premix combustion systems for hydrogen-rich fuels in order to further reduce emissions and to minimize costly diluents. Such systems can involve a high degree of premixing. Unfortunately, hydrogen-rich fuels can be so reactive that significant modifications may be desired in order to safely and cleanly burn these fuels. The modifications (for example, increasing burner velocity, using very high fuel jet velocities), however, can be incompatible with the specifications of modern gas turbine burners (low burner pressure loss, low fuel pressure loss).
Introducing H2-rich fuels into air in order to attain a good air/fuel mixture prior to combustion can be exemplified by FIG. 1, which shows laminar flame speeds for CH4 (a standard gas turbine fuel) and for various H2/N2 mixtures. H2-rich laminar flame speeds can differ from their CH4 counterparts in that:
The peak flame speed can be at least 6 times higher; the flame speed in the entire range of usable fuel/air mixtures can be higher than for CH4; and the peak flame speed can occur at the much lower air excess factor (λ) of approx. 0.6, rather than approx. 1.0.
Turbulent burning velocity can largely determine the flame location in a real burner. This parameter can exacerbate the situation for H2-rich fuels, given that the turbulent burning velocity is a function of pressure for H2 but not so for CH4.
When fuel is injected into hot air, the region near the injection point can be characterized by very poor mixing. On a local scale, λ can vary between 0 and infinity.
Natural Gas:
The flammability limits can be narrow. On the rich side, a flame cannot be sustained, even at relatively high λ (≈0.7 in FIG. 1). The burning velocity (and hence laminar flame speed) can be low, for example, near the rich extinction limit. The risk of ignition in the injection area can be low, and there can be insufficient anchoring in the event of flashback (i.e., the flame is blown off).
H2-Rich Fuels:
The flammability limits can be wide, with very rich mixtures (λ<0.3) capable of sustaining a flame. The burning velocities (and hence laminar flame speeds) can be high. Unfortunately, the peak reactivity of H2-rich fuels can also be in the rich region (for example, around λ=0.5), which can mean that the risk of ignition in the injection area can be high, and the flame anchoring (once flame jump occurs) can be very strong. Flashback thus can result in permanent flame anchoring, which can lead to high emissions and possibly also to hardware destruction.
Known methods of dealing with such high burning velocities, and the drawbacks thereof, are listed below.
Utilizing Dilution:
At any given mixing quality, this action can reduce the burning velocity (see dotted double arrow A in FIG. 1) but not sufficiently. Furthermore, this action does not shift the equivalence ratio at which peak burning velocities occur. Excessive dilution can result in high fuel pressure losses and additional costs. The diluent is not free. In the case of N2, its pressure should be increased from that of the air separation unit to that of the fuel. In the case of steam, there is a loss of efficiency associated with extracting steam from the steam cycle.
Significantly increasing the burner air velocity. In order to be effective, the burner velocity should be increased by a suitable amount, thereby resulting in larger pressure losses across the burner and hence reduction in gas turbine efficiency. Furthermore, such high burner velocities can be incompatible with the standard backup fuels (for example, Natural Gas). It is noted that there will be regions of lower air velocity (for example, boundary layers), which are often near those locations from which fuel is injected.
Injecting fuel at higher velocities in order to avoid flame-holding. Excessive jet velocities can cause high-pressure losses in the fuel system, resulting in higher costs. Approaching the sonic limit also poses stability problems.
None of these known methods, however, address the high flammability of very rich fuel/air mixtures.